Rolling strips of a profile varying lengthwise is one of the most complicated processes of metal press working, the difficulties mainly arising because of the need to produce blanks with accurate dimensions, especially as regards height and width of the section at various points lengthwise of the blank in view of a transient process of metal deformation. With transient rolling processes which are characteristic of the manufacture of strips of a profile varying lengthwise, forward slip and spreading of metal are variables which vary over a broad range not only depending on the taper angle, but also on the character of a change (decrease or increase) in reduction. All these factors cause the need for maximum possible stabilization of metal rolling conditions.
In addition, stringent requirements are imposed upon blanks for few-leaf springs as regards identity of properties over the whole length and presence of open metal texture along the longitudinal axis of the blank which have a special effect on reliability and durability.
Known in the art is a method for making strips of a profile varying lengthwise, in particular, blanks of few-leaf springs, comprising heating a billet, preliminarily reducing it in wedge-shaped dies edgewise on both ends of the billet on a movable table and rolling a varying profile first of one run of the billet during the forward stroke of the table and then the other run during the return stroke of the table between two driven rolls, the upper roll being vertically reciprocatable.
An apparatus for carrying out this method comprises wedge-shaped dies for preliminarily reducing the billet edgewise on both ends, a movable table for supporting the billet, a roll stand having a pair of driven workrolls, the upper roll being reciprocatable, and a means for moving the upper roll.
However, with this method and apparatus for making strips of a profile varying lengthwise, in particular, blanks of few-leaf springs, it is necessary to have sophisticated electronic servo systems so as to ensure accurate dimensions of the strip, especially its thickness because of an inadequate rigidity of the means for vertically moving the upper roll. The provision of the means for moving the upper roll and for supporting the billet on the table substantially complicates design of the roll stand. The need to perform two rolling operations (during forward and return strokes of the table) to form a two-sided taper on the strip prolongs the production process. In addition, the runs of the spring may be of different properties with respect to its central zone because they are rolled consecutively and generally under different temperature conditions.
Also known in the art is a method for making blanks of a profile varying lengthwise, in particular, blanks of few-leaf springs, comprising heating a billet, placing it on a groove of a movable sizing former, feeding the former carrying the billet to a deformation zone, and rolling the billet by driven rolls.
According to the prior art method, a blank of a profile varying lengthwise is manufactured in a single pass in one direction on a sizing former having a groove configured to correspond to the shape of the blank formed after the rolling. To compensate for a spread occurring in rolling a varying profile with this method, the heated billet is preliminarily reduced widthwise in vertical driven rolls before placing on the sizing former.
It is, however, difficult to ensure accurate dimensions of the finished product with this method because of forward slip and spread of metal which vary depending on the amount of reduction. As the rolling process occurs in one direction only and begins with maximum reductions, the metal volume redistributed lengthwise of the billet in the direction from one end of the blank to the other substantially increases to cause a substantial growth of specific pressures upon the rolls and their elastic recoil. This calls for the need to synchronize linear velocities of the rolls and former which can be made by providing auxiliary means so as to complicate the process and make it more expensive.
In addition, both rolling of the billet along the whole length and separate consecutive rolling of its runs cause non-uniformity of properties of the finished product because it is not possible to maintain constant temperature conditions of the rolling and ensure high output.
Therefore, the analysis of prior art methods and apparatuses for making blanks of a profile varying lengthwise shows that these methods and apparatuses are rather complicated and difficult in manufacture; they call for the employment of unwieldy equipment. The use of various designs of the rolls, screwdowns for adjusting the nip between rolls, and the like substantially complicates the process, and employment of electronic servo systems for ensuring accurate dimensions materially raise the cost of equipment and restricts the field of its commercial application.